1. Field of the Invention
The present invention relates to a photomask blank for manufacturing a lithography mask such as a photomask to be used in a lithographic process in the manufacture of a semiconductor device, and more particularly to a halftone type phase shift mask blank which is particularly suitable for a KrF excimer laser, an ArF excimer laser and an F2 excimer laser and a method of manufacturing the halftone type phase shift mask blank.
2. Description of the Related Art
In recent years, it has been apparent that an increase in a resolution and the maintenance of a focal depth which are two important characteristics required for a photolithography are contrary to each other and a practical resolution cannot be enhanced by an increase in the NA of the lens of an exposing apparatus and a reduction in a wavelength (Monthly Semiconductor World 1990. 12, Applied Physics Vol. 60, November (1991)).
Under such circumstances, attention has been paid to a phase shift lithography as a photolithographic technique in a next generation and a part thereof has been used practically. The phase shift lithography is a method of enhancing the resolution of an optical lithography by changing only a mask without a variation in an optical system, and serves to give a phase difference between exposed lights transmitted through a photomask, thereby rapidly enhancing a resolution by utilizing a mutual interference of the transmitted lights.
The phase shift mask uses light intensity information and phase information together and various types such as a Levenson type, an auxiliary pattern type and a self-alignment type (an edge enhancement type) have been known. These phase shift masks have more complicated structures and require a more advanced technique for manufacture as compared with a conventional photomask having only the light intensity information.
As one of the phase shift masks, recently, a phase shift mask referred to as a so-called halftone type phase shift mask has been used practically.
Since the halftone type phase shift mask has both a shielding function of substantially shielding an exposed light by a light semitransmitting section and a phase shift function of shifting (usually inverting) the phase of a light, it has a feature that a shielding film pattern and a phase shift film pattern do not need to be formed separately, and a structure is simplified and manufacture can also be carried out easily.
In the halftone phase shift mask, a mask pattern is processed at a dry etching step. In a method of implementing the shielding function and the phase shift function by separate layers, it is necessary to carry out advanced control for obtaining an excellent pattern shape for both the layer having the shielding function and the layer having the phase shift function. On the other hand, it is possible to use the single etching step by constituting the single-layered light semitransmitting section having both the shielding function and the phase shift function. Consequently, a process for manufacturing a mask can be simplified and an excellent pattern shape can easily be obtained.
As shown in FIG. 1, the halftone type phase shift mask constitutes a mask pattern to be formed on a transparent substrate 100 by a light transmitting section (a transparent substrate exposing section) 200 for transmitting a light having such an intensity as to substantially contribute to exposure and a light semitransmitting section (a shielding section and phase shifter section) 300 for transmitting a light having such an intensity as not to substantially contribute to the exposure (FIG. 1(a)), the phase of the light transmitted through the light semitransmitting section 300 is shifted to have such a relationship that the phase of the light transmitted through the light semitransmitting section 300 is substantially inverted from the phase of the light transmitted through the light transmitting section 200 (FIG. 1(b)), and lights transmitted through the vicinity of the boundary part of the light semitransmitting section 300 and the light transmitting section 200 and going around the other mutual regions by a diffraction phenomenon are set to be cancelled from each other and a light intensity in the boundary part is set to be almost zero, thereby enhancing a contrast, that is, a resolution in the boundary part (FIG. 1(c)).
Moreover, a light semitransmitting section and a light semitransmitting film (a phase shift layer) in the halftone type phase shift mask or blank are to have a sufficient durability to a pretreatment such as washing in a mask manufacturing process and washing in the use of the mask or an acid solution such as sulfuric acid to be used as a washing liquid and a sufficient durability to an alkaline solution such as ammonium.
Referring to a phase shift mask capable of implementing these desired optimum characteristics by a single-layered light semitransmitting section, a proposal for a molybdenum silicide oxide nitride film (JP-A-6-214792 and Japanese Patents Nos. 2878143 and 2989156) has been made.
When the wavelength of a laser to be used for exposure is reduced from i rays (365 nm) or a KrF excimer laser (248 nm) to an ArF excimer laser (193 nm), the deviation of a phase angle with respect to an amount of a change in the thickness of a film is increased. Consequently, it is necessary to enhance a durability to the acid solution and the alkaline solution of the phase shift mask according to the reduction in the wavelength of the exposure.
When the wavelength of the laser to be used for the exposure is reduced, moreover, the energy of a laser beam is increased. Consequently, there is a problem in that the damage of the light semitransmitting section caused by the exposure is increased and a transmittance and a phase difference which are set are deviated for a period of the lifetime of use which is required for the phase shift mask.
On the other hand, the wavelength of an exposing light source to be used for a lithography has been reduced and the NA of the lens of an exposing apparatus has been increased with the microfabrication of a semiconductor circuit. However, the increase in the NA of the lens of the exposing apparatus and the maintenance of a focal depth are contrary to each other. In order to maintain the focal depth, a flatness has been required for a photomask more strictly. In recent years, a flatness of 0.3 to 0.5 μm has been required. The flatness of the photomask depends on the bending strength of a transparent substrate, the flatness of the substrate which is obtained before the formation of a film, and the internal stress of a film for forming a circuit pattern. In particular, the internal stress of the film for forming the circuit pattern has become a serious problem.
Under such circumstances, in the application (Japanese Patent Application No. 2001-246080) filed by the applicant, the density of a light semitransmitting section is increased in order to enhance an acid resistance, an alkali resistance and an excimer laser irradiation resistance of the phase shift mask. This application has disclosed a method of dropping a pressure in an atmosphere including argon and a reactive gas to be used for forming a film constituting the light semitransmitting section by sputtering in order to increase the density of the light semitransmitting section.
Furthermore, the application has disclosed a method of relieving the compressive stress of a light semitransmitting film by carrying out a heat treatment after the formation of the light semitransmitting film based on the fact that the internal stress of the film is increased with a reduction in the pressure in the atmosphere for the execution of the sputtering.
On the other hand, in the application, a heat treatment is to be carried out at a very high temperature (for example, 600° C.) in order to obtain a predetermined stress in some cases. Furthermore, there has been required a method capable of efficiently relieving a compressive stress by the heat treatment.